THE CIRCULATION OF THE BLOOD 



It has already been shown, page 62, that the muscular fibers of the 

 heart differ in structure from skeletal muscle fibers on the one hand, and 

 from unstriped muscle on the other, occupying an intermediate position 



A.& 



FlG - '68. FlG> I69 . 



FIG. 168. The Heart of a Frog (Rana esculenta), from the Front. V, Ventricle; Ad 

 ?E k *? ' auricle; B, bulbus arteriosus, dividing into right and left aorte 



FIG. 169. The Heart of a Frog (Rana esculenta), from the Back. s. v., Sinus venosus 

 opened; c. s. s., left vena cava superior; c. s. d., right vena cava superior; c. i., vena cava 

 mfenor; y. p., vena pulmonales; A. d,, right auricle; A. s. left auricle; A. p., opening of 

 communication between the right auricle and the sinus venosus. X 2^-3. (Ecker.) 



between the two varieties. The heart muscle, however, possesses a prop- 

 erty which is not possessed by skeletal muscle, or by unstriped muscle 

 to such a degree, namely, the property of contracting rhythmically. 



Rhythmicity. The property of rhythmic contraction is shown by the 

 action of the heart within the body; its systole is followed by its diastole in 



FIG. 170. Automatic Contractions of Sinus Muscle from the Terrapin's Heart in 0.7 

 per cent. Sodium Chloride. Time in minutes. (New figure by L. Frazier.) 



regular sequence throughout the life of the individual. The force and fre- 

 quency of the systole may vary from time to time as occasion requires, but 

 there is no interruption to the action of the normal heart or any interfer- 



